A 70 year old male patient presented to the emergency depament with pain in epigastrium and difficulty in breathing for 6 hours. On examination his hea rate was 56 per minute and the blood pressure was 106/60 mm Hg. Chest examination was normal. The patient has been taking omeprazole for gastroesophageal reflux disease for last 6 months. What should be the initial investigation:

**Core Concept**
Omeprazole is a proton pump inhibitor (PPI) that can cause hypochlorhydria, a state of reduced gastric acid secretion. This reduction in gastric acid can lead to a decrease in the absorption of vitamin B12, a condition known as vitamin B12 deficiency. However, this is not the primary concern in this patient. A more immediate concern is the potential for PPI-induced hypomagnesemia, which can cause hypocalcemia, leading to neuromuscular excitability and potentially life-threatening arrhythmias.

**Why the Correct Answer is Right**
The patient's presentation of pain in the epigastrium and difficulty breathing, along with a low heart rate (bradycardia) and normal blood pressure, is suggestive of hypocalcemia. The prolonged use of omeprazole can lead to hypomagnesemia, which in turn can cause hypocalcemia. Hypocalcemia can cause neuromuscular excitability, leading to tetany, which can manifest as muscle cramps, spasms, or even seizures. The initial investigation should focus on confirming the diagnosis of hypocalcemia.

**Why Each Wrong Option is Incorrect**
**Option A:** Serum gastrin levels would not be the initial investigation of choice in this patient, as the primary concern is not gastrinoma or Zollinger-Ellison syndrome.
**Option B:** Serum gastrin levels would not be the initial investigation of choice in this patient, as the primary concern is not gastrinoma or Zollinger-Ellison syndrome.
**Option D:** Serum gastrin levels would not be the initial investigation of choice in this patient, as the primary concern is not gastrinoma or Zollinger-Ellison syndrome.

**Clinical Pearl / High-Yield Fact**
Prolonged use of PPIs can lead to hypomagnesemia, which can cause hypocalcemia, especially in patients with a history of malabsorption or those taking other medications that affect magnesium levels. Remember to check serum magnesium levels in patients taking PPIs for more than 6 months.

**Correct Answer:** C. Serum calcium levels

✓ Correct Answer: A. An ECG